The initial application for semiconductor lasers was in fiber optic communications which required only relatively low laser output powers: 3 to 5 mW delivered in a single stable beam corresponding to lowest order mode operation. Such low power semiconductor lasers have found extensive use in long and short range communications systems, and in digital audio disc playback apparatus.
More recently, however, there has been an ever increasing demand for single mode lasers which can emit at significantly higher powers, i.e., powers in excess of 100 mW. Such devices could be used in optical recording, high speed printing, data distribution systems, analog signal transmission, long distance optical communication systems at high bit rates, industrial processing, and as pumps for solid state lasers. For all of these applications, it is desirable to have a high powe laser that stably and reliably produces a single well defined beam, i.e., lowest order mode output.
The most important type of semiconductor diode laser is the double hetero-structure (DH). It is the primary type of semiconductor diode laser being used commercially today. The DH comprises a semiconductor body having first and second relatively wide-bandgap cladding layers of opposite conductivity type, and a relatively low doped narrow bandgap active layer which is sandwiched between and contiguous with the wide bandgap cladding layers. The layers are grown on a suitable substrate. Illustratively, the narrow bandgap active layer comprises GaAs and the wide bandgap cladding layers comprise Al.sub.x Ga.sub.1-x As where x is about 0.25 to about 0.35. The substrate is GaAs. Generally, electrical contacts are provided on the top and bottom surfaces of the semiconductor body comprising the laser so that the resulting diode structure can be forward biased.
When the structure is forward biased, electrons and holes from the cladding layers are injected into the active layer where radiative recombination takes place. The radiation occurs within a wavelength band determined by the bandgap of the active layer. The cladding layers are transparent to the emitted radiation. Normally, the active layer is highly absorbing. In this example the emitted radiation is at 0.88 microns. The emitted radiation travels longitudinally back and forth between partially reflective end facets of the semiconductor body. As the forward bias pumping current is increased further, absorption is diminished and finally is replaced by amplification. Lasing begins when the round trip optical gain within the wavelength band for emission exceeds losses due to mechanisms such as absorption, scattering, and facet transmission. When the current begins, the circulating power increases rapidly until the net gain saturates at a value infinitesimally below the net loss. The laser then achieves a steady state circulating power level. Radiation transmitted through the end facets produces the output beam.
The index of refraction of the active layer is larger than the index of refraction of the surrounding cladding layers. Thus, the emitted radiation is transversely confined in a one-dimensional dielectric waveguide formed by the two cladding layers and the active layer. For state of the art devices with active layers thinner than about 0.3 micrometers, the dielectric waveguide is such that only the fundamental transverse mode is supported. (As used herein, transverse means perpendicular to the plane of the layers comprising the laser.)
While the light is guided in the lowest order mode in the transverse direction, such is not normally the case in the lateral direction (i.e., in the plane of the layers). If a wide stripe contact is used to inject the pumping current, the optical output exhibits unstable, multimode and filamentary behavior, such as was the case with early diode lasers. The light versus current characteristic is highly nonlinear. The unstable filamentary behavior is exagerated as one goes to higher and higher powers. Thus, such wide stripe structures, although they can produce the desired high power, have heretofore proven unsuitable for use in the typical applications contemplated for high power diode lasers, which applications require a single, stable, lowest order mode optical beam. However, they have found use where the requirement is simply infrared illumination.
Various techniques have been developed to provide for lateral confinement of emitted radiation, so as to achieve stable and reliable laser operation in the fundamental lateral mode. The simplest technique involves use of a narrow stripe contact (less than 8 um wide) on the upper surface of the laser. If the electric contact is shaped into a narrow stripe running the length of the diode between the facets, the profile of the injected carriers establishes a weak waveguide which provides a type of current dependent guiding commonly referred to as gain guiding. However, these structures exhibit strongly unstable multiple beam behavior at high powers and a non-linear light output versus current behavior known as kinking.
In the last ten years, researchers have concentrated on controlling the lateral modes in diode lasers by introducing dielectric waveguide structures in the lateral direction as well as the transverse direction. These lasers are known as index guided lasers. Using index guiding techniques and thin active layers, it is possible to produce a laser which supports only the fundamental transverse mode and the fundamental lateral mode. Such lasers, having a two-dimensional waveguide structure, emit a single, spatially coherent beam of light whose intensity profile at the facet is a bell-shaped surface. The single beam will remain stable in the laser's far field (more than 5 microns from the emitting facet) whether the current driving the diode is pulsed or continuous, and independent of the current level short of damaging the device. The output power is highly linear with current.
One type of index-guided laser is known as Buried Hetero-structure. In this structure a stripe-shaped relatively high index active region (e.g. GaAs) is surrounded transversely and laterally by lower index of refraction material (e.g. AlGaAs). Proper choice of dimensions assures lowest order mode operation. Another type of index guided laser is known as the ridge-guided structure in which a longitudinally extending ridge is etched into the upper cladding layer of a double hetero-structure laser. Light propagating in the active layer tends to be laterally confined to the region below the ridge because of the effective decrease in index of refraction associated with the boundary of the ridge. Other index guided structures are described in Botez, "Laser Diodes Are Power Packed", IEEE Spectrum, June 1985, pp. 43-54.
However, the output power of such index guided lasers formed using the AlGaAs-GaAs materials system is limited by damage to the partially reflecting end facets of the diode laser. If an AlGaAs diode emits continuous wave optical power densities in excess of 6 to 9 mW per square micrometer of emitting area at the facet, the internal laser power density becomes so high that chemical reactions occur at the partially reflective end facets from which the light emerges. Stress is created and the end facet regions gradually darken, absorbing light, and the laser performance degrades. The output power also exhibits rapid time variations associated with the dynamics of the absorption process.
In addition, laser light is absorbed because of the non-radiative recombination of carriers at the end facets, where the boundary of the semiconductor material has a high density of surface states. At high optical power densities (20-25 mW per square micrometer), heavy radiation absorption at the facets induces a thermal runaway process, which causes the mirrors to melt, thus causing catastrophic failure of the diode laser. The output power limits imposed by the gradual or catastrophic failure of the laser end facets have heretofore been extended by any of three known techniques.
First, the size of the lasing spot can be increased both perpendicular to and parallel to the plane of the layers, to spread the emitted power over a wider area, thus allowing operation at higher powers before the emitting facets are gradually degraded or catastrophically damaged. Transverse spreading is limited in a double heterostructure because a thin active layer is needed to achieve fundamental transverse mode behavior and there are practical limits on how thin the layer can be. Furthermore, while lateral spreading may lead to substantially increased power, it may also lead to the appearance of higher order lateral modes. Thus, merely increasing the area of the laser spot as, for example, by increasing the width of the active region in a stripe-shaped structure will not lead to the achievement of a high power laser diode which operates in the fundamental lateral mode.
The second technique for boosting the power capability of a diode laser is to apply an anti-reflection coating to the front facet and a reflection coating to the back facet of the laser to increase the ratio of the laser's emitted power to internal power, thereby getting more power out for a given amount of internal power. This technique has limited value since in a typical laser 2/3 of the internal power is transmitted through the facets, even without an AR coating. A third technique for increasing the power output of a laser is to prevent the mirror regions from absorbing laser light or experiencing non-radiative recombination. Such structures are known as non-absorbing mirror or NAM structures. These are effective in increasing the catastrophic damage limit but are less efficient than conventional lasers.
The aforementioned structures have proved to be of limited usefulness in achieving reliable fundamental transverse and fundamental lateral mode behavior at high powers.
In view of the foregoing, it is the object of the present invention to provide a reliable, high power semiconductor diode laser which operates in the fundamental lateral mode. More particularly, it is the object of the present invention to provide a high power semiconductor diode laser by increasing the lateral size of the laser spot while at the same time avoiding higher order lateral modes. In other words, it is the object of the present invention to provide a wide stripe semiconductor laser diode in which higher order lateral modes are suppressed so that stable and reliable operation in the fundamental lateral mode is achieved.